Dear @Jason.Jonkman,
I wanna ask you about the overturning moment (in pitch and roll) which is important for platform stability. I did not succeed in finding it from the Excel file “Outlist Parameters”. Do you confirm that this output does not exist ?
Thank you in advance
Best Regards,
Riad
Dear @Riad.Elhamoud,
Are you referring to OpenFAST outputs such as ElastoDyn output TwrBsMyt, which provides the tower-base fore-aft bending moment, or HydroDyn output HydroMyi, which provides the integrated hydrodynamic pitching moment?
Best regards,
Dear @Jason.Jonkman,
Thank you for your answer and sorry for the late reply.
I think the overturning moment is the sum of this two moments. However, i do have some inquiries:
1- The TwrBsMyt is the moment at towerbase which is at the interface between the floater (transition piece) and the tower, do you confirm? TwrBsMyt is due to the aerodynamic loads distributed along the height of the tower + the thrust force induced at the hub multiplied by the lever arm (i.e. tower height), do you confirm?
2- HydroMyi is calculated by multiplying the resultat hydrodynamic load by the lever arm and this moment is computed at a user-defined specific point. This point is defined through ***NWaveElev ***
WaveElevxi, WaveElevyi in HydroDyn file, do you confirm ? But, i cannot find where to specify the elevation. In other words, let’s assume that i want HydroMyi at the location (0,0,-4), plz could you tell how to specify the elevation -4 to HydroDyn ?
Thank you in advance,
Best Regards,
Riad
Dear @Riad.Elhamoud,
Regarding (1), I agree with the addition that TwrBsMyt also includes the effect of turbine weight (including overturning moment from the mass offset relative to the tower base) and inertia from turbine motion (if the turbine is accelerating).
Regarding (2), HydroMyi is not computed about the point defined by (WaveElevxi,WaveElevyi,0). Rather, HydroMyi is computed about (0,0,0) in the body-fixed coordinates of the hydrodynamic body and there is no input to add an offset to this output.
Best regards,
Dear @Jason.Jonkman,
Thank you for your answer. Now, it is clear for me.
Excuse-me, i have one additional question: Does OpenFAST compute stiffness for platform pitch ?
I mean by platform pitch stiffness, a scalar K_ptfm_pitch in such a way that the pitch moment denoted
by M_ptfm_pitch = K_ptfm_pitch * ptfm_pitch
where ptfm_pitch is platform pitch in radians.
Thank you a lot.
Best Regards,
Riad
Dear @Riad.Elhamoud,
Certainly there is stiffness/restoring in pitch in OpenFAST, as a result of hydrostatics, system weight, moorings, etc. While your variable K_ptfm_pitch is not specified directly, the contribution of each of these effects will contribute to this stiffness.
Best regards,